Downhole cable with reduced diameter

ABSTRACT

An optical/electrical cable for downhole environments includes a plurality of optical fibers disposed within an interior metal tube. An electrically conducting layer surrounds the interior metal tube, an insulation layer surrounds and contacts the electrically conducting layer, and an exterior metal tube surrounds and contacts the insulation layer.

DESCRIPTION Technical Field

The present disclosure relates generally to an optical/electrical cable,and more particularly, to a cable for downhole applications having aplurality of optical fibers encased in a metal tube and an electricallyconductive layer coaxially surrounding the metal tube for conductingelectrical current.

Background

Several types of cables exist for monitoring environmental conditions,sending communications and providing electrical power within a hazardousenvironment, such as a downhole application in an oil or gas well. Onetype is a “tube encased conductor” (TEC) cable. A TEC cablecharacteristically has an electrical conductor disposed within a metaltube for protecting the conductor from the hazardous environment. TECcables are used for transmitting electrical power to various devices atthe distal end of the cable that monitor conditions in the downholeenvironment, such as temperature and pressure.

Another type of cable for hazardous environments is a “tube encasedfiber” (TEF) cable. Similar to TEC cables, TEF cables protect an opticalfiber from the environment by disposing the fiber within a small metaltube (referred to as “fiber in metal tube” or “FIMT”). TEF cables may beused in oil and gas wells either as fiber waveguides to transfer datafrom downhole tools or as sensors for distributed temperature sensing(DTS), distributed acoustical sensing (DAS), and other sensingapplications.

When the cable contains both electrical conductors and optical fibersencased in one or more tubes, the cable is referred to as a TEC/TEFcable. Commercially available TEC/TEF downhole cables can have a ¼ inch(e.g., 6.35 mm) diameter steel tube as an outer casing. It contains aninsulated conductor and another steel tube encasing an optical fiber.TEC/TEF cables of ¼ inch diameter are typically used for permanentinstallation in an oil well.

As known from, for example, the internet sitehttp://petrowiki.org/Coiled_tubing_drilling, coiled tubing surveys andcoiled tubing drilling are also common applications for downhole cables.The term “coiled tubing” refers to a long metal tube, normally 2.5 cm to8.25 cm in diameter, which is spooled on a large reel. The tube is usedfor interventions in oil and gas wells and sometimes as productiontubing in depleted gas wells. The term “coiled tubing drilling” refersto a combination of coiled tubing and directional drilling (e.g.,drilling non-vertical wells) using a mud motor to create a system fordrilling reservoirs.

An emerging technique for coiled tubing is to profile an oil well withdistributed temperature sensor (DTS) or data acquisition system (DAS)techniques using fiber optic sensors within a TEF cable. Small diameterTEF cables, i.e., cables having an outer diameter of no more than 4 mm,can be injected into the coiled tubing string before the coiled tubingis lowered into the oil or gas well.

To further enhance the well profile, electromechanical pressure sensorsmay be used when sensing with coiled tubing. Those sensors, however,require electrical current not provided by a TEF cable. The stainlesssteel tube as part of the FIMT within a TEF cable generally does nothave sufficient current carrying capacity because its resistance is toohigh. Commercially available TEC/TEF cables, which do provide both fibersensing and electrical current, are too large and heavy for deploymentin coiled tubing with their ¼ inch (6.35 mm) diameters. Fieldexperiences with TEF cables for coiled tubing deployments havedemonstrated that the largest outer diameter for sensing cables that canbe used is 4 mm.

U.S. Pat. No. 8,295,665 (the '665 patent) discloses a downhole hybridtype cable including a center fiber/gel filled stainless steel tube witha copper wire wrapped around the tube and an insulation layer around thecopper wire tube. The copper wire is disposed in the helical spaceformed by the metal tube. The metal tube and the copper element are putinto a metallic tube. The metallic tube has a ¼ inch diameter (6.35 mm).

U.S. Pat. No. 5,493,626 (the '626 patent) discloses a downholeelectrical/optical instrument cable for use in a well logging system forhigh-pressure environments. The cable includes a single, hermeticallysealed optical fiber for signaling surrounded by layers of protectivematerial and a gel and encapsulated by a protective sheath. The sheathmay be a laser-welded metal tube. A layer of electrical conductorsbetween an optional inner insulator and an outer insulator layersurrounds the protective tube and is formed of braided copper ratherthan a helical “serve” of copper. A plurality of strength member strandssurrounds the outer insulator layer. The strength members include aninner layer of stainless steel strands wound helically around the outerinsulator layer in one direction, and an outer layer of stainless steelstrands wound helically around the inner layer of strength memberstrands in an opposite serve or winding. The strands and the copperbraid layer are conductive and can provide an electrical power supplyloop. The total diameter of the cable is approximately 5.77 mm, but itcan vary within a range from about 4.76 mm to about 7.94 mm.

U.S. Pat. No. 8,931,549 (the '549 patent) discloses a cable for welllogging in marine-submersible and subterranean oil and gas wells. The'549 patent concludes that conventional logging cables with wrappedsteel wires and solid copper conductors are not sufficient for deepoffshore wells. The disclosed cable includes at least one optical fiberencapsulated in a polymeric material wherein the optical fiber cable isloosely disposed inside a beryllium alloy tube. The beryllium alloytube, which is conductive, is encapsulated in an amorphous dielectricmaterial, which is further encapsulated on its outer surface by anamorphous polymeric electrically conductive material. This outer layercan be zinc, tin, or other material wrapped, sputtered, or doped thesurface to form a shield for both mechanical and electromagneticeffects.

PCT International Publication WO 2009/143461 (the '461 publication)discloses cables for use in a downhole environment, such as in oil orgas wells for conveying well logging tools. In particular, the '461publication discloses a cable including a cylindrical central corepreferably formed of a communication element capable of carrying datasignals, such as an optical fiber, which may be encased in a protectivemetal tube. The cable then includes concentric layers intended toprotect a polymer fiber layer in the protective tube. It is preferredthat at least one, and possibly both, of inner and outer layers aroundthe polymer fiber layer be formed of a solid electrical conductor, suchas a metallic conductor. A layer inner to the polymer fiber layer may beunnecessary if the core is designed to eliminate gas, water, andcorrosive migration up and down the core by adding a “water block” agentor fluid. In the event that either of the inner layer and outer layer isnot formed of a metallic material, then that layer will preferably beformed of a plastic material such as polyether ethyl ketone (PEEK), oranother high density polypropylene. Outer protective sheath will againpreferably be formed of PEEK, or another plastic material havingexceptional resistance to abrasion, temperature and invasive materials.The '461 publication discloses that cables with an outer diameter ofroughly between 0.3 inch and 0.5 inch (about 7-13 mm), will benefit mostfrom this construction.

European Patent Publication EP 0945876 (the '876 publication) disclosesa hybrid cable for installation in conduits for fluid media (for examplein waste water, fresh water, or gas lines), with at least one opticalwaveguide arranged in a protective sheath, one or more electricalconductors, and a jacket surrounding the electrical conductor and theprotective sheath. In particular, the '876 publication discloses ahybrid cable with two concentric metal tubes, of which at least theexternal one is corrugated. An insulating layer separates the two metaltubes, and a jacket of polyethylene surrounds the external tube. Theinner tube surrounds a plurality of optical fibres. In many cases,conductivity of the inner metal tube can be sufficiently achieved bycoating its surface with a metal of high conductivity.

PCT International Publication WO 2015/038150 (the '150 publication)discloses a fiber optic electrical core that may be incorporated into afiber optic slickline (application that is run over a conveyance linethat is substantially below about 0.25 inches, i.e. 6.35 mm, in overallouter diameter). One or more fiber optic threads, each jacketed by aconventional polymeric buffer, may be placed within a welded steel tubein a loose fashion with a sufficiently thick electrically insulatingpolymer layer thereabout, and surrounded by a conductive member. Theelectrically conductive member may also be surrounded by an insulatingpolymer jacket. To complete the fiber optic slickline, the fiber opticelectrical core may be surrounded by a synthetic fiber layer. Adherencebetween a subsequent cladding layer and the fiber synthetic layer may beenhanced by way of the intervening adherent layer. Cladding layer may bea conventional metal-based layer such as a steel jacket.

Applicant has faced the problem of providing a TEC/TEF downhole cablewith minimal diameter, particularly with a diameter of no more than 4 mmsuitable for coiled tubing drilling applications. Those cables need tobe capable of insertion into a coiled tubing string before the coiledtubing is lowered into the oil or gas well. TEC/TEF cables designed forwell logging applications are too large, typically having an outerdiameter of 6.35 mm, which can be difficult to insert in a coiled tubingand for limited length (not greater than 3 km). While some TEF cableshaving diameters within 4 mm are known, those small-diameter TEF cablesdo not have electrical conductors, nor do they have sufficient currentcarrying capacity due to the high electrical resistance of theirstainless steel tube.

SUMMARY

To provide a sufficient current carrying capacity within the smallspace, Applicant provides the conductor in the form of an electricallyconducting layer coaxially surrounding the metal tube that encases theoptical fibers. With the coaxial design, both the conducting layer andthe optical fibers can be fit within a small diameter (e.g., no morethan 4 mm) cable that meets size requirements of coiled tubing drilling.The coaxially configured TEC/TEF cable contains a sufficient number ofoptical fibers for distributed temperature sensing (DTS) and distributedacoustic sensing (DAS) along with an electrically isolated conductivepath that transmits electrical power to a downhole tool all in one outertube having a diameter of no more than 4 mm.

Applicant has found that cables for downhole applications having adiameter of no more than 4 mm for use in coiled tubing drillingapplications may be attained with a cable structure in which opticalfibers are encased in a welded metal tube, an electrically conductinglayer is disposed coaxially around the metal tube, an insulating layersurrounds the electrically conducting layer, and an outer metal tubesurrounds the insulating layer.

The term “coaxial” used herein refers to the configuration where an axisof symmetry of an inner tube or layer is substantially the same as anaxis of symmetry of an outer tube or layer.

Applicant has found that this coaxial configuration can reduce the outerdiameter of the cables, such that the cables can meet size requirementsfor coiled tubing drilling applications, while also providing sufficientcurrent carrying capacity for electronic equipment at the distal end ofthe cable. Simultaneously, the cable includes one or more optical fibersconfigured for data communication and/or measurement or sensing ofenvironmental parameters, such as temperature, pressure, strain, etc.The optical fibers are disposed within a metal tube (e.g., FIMT).

Applicant has found that the cable with a reduced diameter can providesufficient current capacity with an electrically conducting layer (alsoreferred to as a conducting layer or conductive layer) in the form of atape (e.g., wrapped tape), longitudinally welded foil, or tube. Theconducting layer may include a single layer or a plurality of layers.The conducting layer wraps around the metal tube that encases theoptical fibers, and hence is coaxial to the metal tube.

The conducting layer is surrounded by and in direct contact with anelectrically insulating layer (also referred to as an insulating layeror insulation layer). The insulation layer is in turn surrounded by anouter metal tube. The coaxially disposed conducting layer and FIMT, asopposed to discrete electrical conductors or thick copper braids, allowsfor the reduction of the total diameter of a standard 6.35 mm TEC/TEFcable down to an overall diameter of about 4 mm, or possibly smaller ifa lower current carrying capacity is sufficient. In addition, Applicanthas identified that a cable with the disclosed structure avoids the needto strand the FIMT and a conductor, simplifying the manufacturingprocess.

Accordingly, in one aspect, an optical/electrical cable for downholeenvironments consistent with the disclosed embodiments comprises aplurality of optical fibers optionally embedded in a gel and disposedwithin a first interior metal tube (also referred to as a first metaltube or an interior metal tube). The cable also includes an electricallyconducting layer surrounding the first metal tube. An insulation layersurrounds and contacts the electrically conducting layer, and a secondexterior metal tube (also referred to as a second metal tube or anexterior metal tube) surrounds and contacts the insulation layer. Theexterior metal tube has an outer diameter of 4.0 mm at most.

In some embodiments, the interior metal tube has an outer diameter ofabout 1.8 mm. It can be made of stainless steel. In some embodiments,the exterior metal tube has a thickness of about 0.56 mm. It can be madeof a steel alloy. The interior metal tube and the electricallyconducting layer can have a combined thickness of about 0.28 mm.

In some embodiments, the electrically conducting layer in the form of atape is helically wound or cylindrically wrapped around the first metaltube. In some embodiments, the electrically conducting layer includescopper. In some embodiments, the electrically conducting layer is in theform of a tube made of copper. In some embodiments, the electricallyconducting layer is a foil formed into a copper tube and longitudinallywelded along its seam. The electrically conducting layer can have anouter diameter of about 2.05 mm.

A separating layer can be provided between the electrically conductinglayer and the first metal tube. The separating layer can be apolyethylene terephthalate tape.

In some embodiments, the insulation layer includes at least one ofpolypropylene, fluorinated ethylene propylene (FEP), perfluoroalkoxy(PFA), Ethylene ChloroTriFluoroEthylene (ECTFE), EpitaxialCo-Crystallized Alloy (ECA).

In another aspect, a TEC/TEF cable with a coaxial construction includesan exterior metal tube, an insulator, a layered tubular conductor, and aplurality of optical fibers. The exterior metal tube can have an outerdiameter of no more than 4.0 mm and a thickness sufficient to protectthe interior of the tube from external environmental conditions in adownhole. The layered tubular conductor comprises the electricallyconducting layer surrounding and in direct contact with the interiormetal tube, where the electrically conducting layer has an electricalconductivity higher than the interior metal tube. The layered conductorcan have a thickness of about 0.28 mm. The layered conductoradvantageously has a composition sufficient to conduct up to 1 ampere ofcurrent at 600 volts or less. The insulator separates the layeredtubular conductor from the exterior metal tube. The optical fibers arehoused within the interior metal tube.

The optical fibers can perform as optical sensors or as communicationoptical fibers. The interior metal tube can house together both one ormore optical fibers performing as an optical sensor and one or moreoptical fibers performing as a communication optical fiber.

For the purpose of the present description and of the appended claims,except where otherwise indicated, all numbers expressing amounts,quantities, percentages, and so forth, are to be understood as beingmodified in all instances by the term “about,” if not already modified.Also, all ranges include any combination of the maximum and minimumpoints disclosed and include any intermediate ranges therein, which mayor may not be specifically enumerated herein.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosed embodiments will be now described more fully hereinafterwith reference to the accompanying drawing, in which some, but not allembodiments of the invention are shown. The drawing illustrating theembodiment is a not-to-scale schematic representation.

The sole FIGURE shows a schematic cross-sectional view of a cable,consistent with disclosed embodiment.

DESCRIPTION OF THE EMBODIMENTS

Reference will now be made in detail to the present exemplaryembodiments, examples of which are illustrated in the accompanyingdrawing. The present disclosure, however, may be embodied in manydifferent forms and should not be construed as limited to theembodiments set forth herein.

The sole FIGURE illustrates a cross-sectional view of anoptical/electrical cable or TEC/TEF cable 100, consistent with thedisclosed embodiment. Cable 100 is suitable for downhole applications,such as coiled tubing drilling, in the oil and gas industry. Cable 100is a TEC/TEF cable that includes both optical fibers to function assensors of environmental parameters and/or to transmit data, and anelectrical conductor to transmit power to devices in the oil and gaswells.

Cable 100 includes at least one optical fiber for sensing and/or datatransmission. The example shown in the FIGURE includes two opticalfibers 105 and two optical fibers 110, along with other optical fibersthat are not referenced. Optical fibers 105 and 110 may be any suitableoptical fibers depending on the temperature rating of cable 100 fordownhole applications. Optical fibers 105 and 110 may be the same typeof optical fibers, or different types of optical fibers. In the exampleshown in the FIGURE, two optical fibers 105 are acrylate coated 50/12585° C. optical fibers, colored blue and orange (different patterns areused for the cross sections of the fibers to schematically representdifferent colors in the FIGURE). Two optical fibers 110 are acrylatecoated single mode 85° C. optical fibers, colored green and brown(different patterns are used for the cross sections of the fibers toschematically represent different colors in the FIGURE).

At least one of optical fibers 105 and 110 functions for sensing anenvironmental parameter in an oil and/or gas well. Environmentalparameters may include temperature, pressure, and/or acousticalmeasurements. At least one of optical fibers 105 and 110 functions toprovide data transmission between other sensors or devices deployed downthe well, and a data receiving device (e.g., a computer, a data storagedevice, a monitor display, a signal processor, etc.) deployed outside ofthe well.

The optical fibers are disposed within an inner space defined by a firstinterior metal tube 120 (also referred to as an interior metal tube 120or a first metal tube 120). The space is filled with a gel 115. Theoptical fibers 105 and 110 are embedded within the gel 115. Thecombination of the optical fibers 105, 110, the gel 115, and the firstmetal tube 120 may be referred to as a unit called FIMT (fiber in metaltube). In some embodiments, the outer diameter of the FIMT unit may be2.2 mm.

The gel 115 is any type of gel suitable for the temperature rating ofcable 100 for downhole applications. The gel 115 may be an inert gelthat is injected into the space defined by the first metal tube 120,filling the space around the optical fibers including optical fibers 105and 110. The gel 115 may fix the optical fibers in their positions, andsupport the optical fibers within the first metal tube 120. The gel 115may also functions to mitigate or reduce vibration, shock, friction, andabrasion caused by the external environment to the optical fibers. Thegel 115 may be a viscous material. One example of the gel 115 forfilling the optical fibers is Sepigel™ produced by SEPPIC SA., used withan excess fiber length (EFL) of 0.15% +/−0.05%.

In some embodiments, the first metal tube 120 may be made of anysuitable metal material, such as steel, copper, aluminum, etc. A varietyof materials, including steels and alloys, may be used to make the firstmetal tube 120. Examples of such materials include SS 304, SS 316L,A825, and A625. In one example, the first metal tube 120 is made ofstainless steel SS 304, with a wall thickness of 0.15 mm (or 0.006inch), and an outer diameter of 1.8 mm (or 0.071 inch).

As shown in the FIGURE, cable 100 includes an electrically conductinglayer 125. The electrically conducting layer 125 includes at least oneconductor configured for electrical power transmission. The conductormay be made of any electrically conductive material, such as copper ortinned copper. Preferably, the electrically conductive material has ahigher conductivity than the material of the first metal tube 120, e.g.copper versus stainless steel. Other conductive materials that can beused for the conducting layer 125 include aluminum, gold, silver, etc.The conductor may have any suitable form or shape, such as wire, mesh,tape, tube, strip, etc.

The electrically conducting layer 125 is helically wound around an outersurface of the first metal tube 120, or is cylindrically wrapped aroundthe outer surface of the first metal tube 120. The electricallyconducting layer 125 may directly contact the outer surface of the firstmetal tube 120, or may indirectly contact the outer surface of the firstmetal tube 120 with an additional layer, such as insulation layer,disposed between the outer surface of the first metal tube 120 and theelectrically conducting layer 125.

In some embodiments, the electrically conducting layer 125 is in theform of a tape helically wound on the outer surface of the first metaltube 120. For example, the electrically conducting layer 125 may bemetal strips that are helically wound on the outer surface of the firstmetal tube 120.

In some embodiments, the electrically conducting layer 125 iscylindrically wrapped around the outer surface of the first metal tube120. For example, the electrically conducting layer 125 may be appliedas a foil longitudinally wrapped to surround the outer surface of thefirst metal tube 120. Or the electrically conducting layer 125 may be inthe form of a tube welded foil. In some embodiments, the electricallyconducting layer 125 may be a seam welded tube (e.g., a seam weldedcopper tube). For example, the seam welded tube may have a thickness of0.127 mm (or 0.005 inch) and an outer diameter of 2.05 mm (or 0.081 in).When a seam welded tube is used, the FIMT unit may be made smaller thanwith other configurations for the electrically conducting layer 125.Thus, the use of a seam welded tube may permit an increase in thecable's electrical conductivity and allow for a thicker insulation layer(discussed below) to be used.

The electrically conducting layer 125 may take other forms. For example,the electrically conducting layer 125 may be a continuously welded tube,an extruded metal tube, a braided wire layer, a helically applied layerof fine wires, or any other concentrically applied layer of metal.

The electrically conducting layer 125 is coaxial with the first metaltube 120. That is, the axis of symmetry of the electrically conductinglayer 125 is the same as the axis of symmetry of the first metal tube120. The electrically conducting layer 125 forms the primary conductivepath for power transmission. When an inner surface of the electricallyconducting layer 125 directly contacts the outer surface of the firstmetal tube 120, the first metal tube 120 may also carry a small amountof the total current when the resistance of the material for making thefirst metal tube 120 (e.g. steel) is higher than the resistance of thematerial for making the electrically conducting layer 125 (e.g.,copper).

Therefore, the combination of the interior metal tube and theelectrically conducting layer may form a layered tubular conductor forthe cable. One layer of the conductor, namely, electrically conductinglayer 125 made, for example, of copper, has a higher conductivity thanother layers, namely, the interior metal tube 120 made, for example, ofstainless steel. Preferably, the layered tubular conductor should havethe capacity through its material composition and layer thicknesses toconduct up to 1 ampere of current at 600 volts or less.

Cable 100 also includes an electrical insulation layer 130 coaxiallysurrounding an outer surface of the electrically conducting layer 125.An inner surface of the insulation layer 130 directly contacts an outersurface of the electrically conducting layer 125. The insulation layer130 electrically insulates the electrically conducting layer 125 fromthe outer environment. Materials used for the insulation layer 130depend on the cable temperature rating. Examples of the materials formaking the insulation layer 130 include polypropylene, fluorinatedethylene propylene (FEP), perfluoroalkoxy (PFA), EthyleneChloroTriFluoroEthylene (ECTFE), Epitaxial Co-Crystallized Alloy (ECA).In one example, the insulation layer is made of natural FEP and has anouter diameter of 2.79 mm (or 0.110 inch). The minimum and nominalthickness of the insulation layer can be calculated by the skilledperson in view of the cable voltage rating.

Cable 100 includes a second exterior metal tube 135 (also referred to asa second metal tube 135 or an exterior metal tube 135) coaxiallysurrounding an outer surface of the insulation layer 130. An innersurface of the second exterior metal tube 135 may directly contact anouter surface of the insulation layer 130. The second metal tube 135 maybe made of any suitable metal material, preferably steel or steel alloy.For example, a variety of steels and alloys may be used to make thesecond metal tube 135, such as SS 304, SS 316L, A825, and A625. As shownin the FIGURE, the second metal tube 135 is a single tube coaxiallysurrounding the outer surface of the insulation layer 130. In oneexample, the second metal tube 135 is made of alloy A825 with a wallthickness of 0.55 mm (or 0.022 inch), and an outer diameter (OD in theFIGURE) of 4 mm (or 0.1575 inch).

The outer diameter of the second metal tube 135 (i.e., the totaldiameter of the cable 100) is 4 mm at most. The outer diameter issubstantially the same as the total diameter of the cable 100. Thus, thetotal diameter of the cable 100 is no more than 4 mm. In someembodiments, when lower currents are needed or thinner tubes (e.g.,first metal tube 120, second metal tube 135) are possible, the totaldiameter of the cable 100 may be reduced to be less than 4 mm. It isalso possible to have an outer diameter greater than 4 mm, although suchan embodiment may be limited in its applications for cable tubingsensing in a downhole environment. In the case, the second metal tubecan perform as return or ground conductor without any specificmodification to its design.

In radial outer position with respect to the second metal tube, aprotective jacket (not illustrated) can be provided. The protectivejacket can be made of polymeric material such as polyethylene,preferably high density polyethylene,

The disclosed cable has an electrically isolated conductive path with alow voltage of 600 volts DC or less (e.g., 500 volts DC), and can carrya current of 1 ampere. The disclosed cable can be used for a continuouslength of 5 kilometer (km) or longer. In some embodiments, the disclosedcable can tolerate a maximum temperature of 300° C. The disclosed cablehas a temperature rating of 175° C. (short term) and 150° C. (longterm). In some embodiments, the disclosed cable can have an externalcollapse pressure of 28,900 psi (or about 2.0×10⁸ Pa) and a cable weightof 71 kg/km (or 48 lbs/1000 ft). The disclosed cable can have a DCresistance of 21.9 ohms/km (or 6.66 ohms/1000 ft) at 20° C. for a seamwelded copper tube as the electrically conducting layer 125 and astainless steel tube for FIMT 120.

The optical fibers within the cable can function as sensing fibers andas communication fibers. In some embodiments, the attenuation is ≦3.5dB/km for multimode at 850 nm, 1.5 dB/km for multimode at 1300 nm. Theattenuation is ≦0.7 dB/km for single mode at 1310 nm, and ≦0.7 dB/km forsingle mode at 1550 nm.

The disclosed cable can meet typical downhole application requirements,such as, for example, maximum pressure of 6.89×10⁷ Pa (or 10,000 psi),and maximum temperature of 150° C.

The disclosed cable can be used as a lower profile “heatable” fiberoptic sensing cable. The term “heatable” downhole cables refers to atechnique where electrical conductors are heated for a temporary periodand the cooling rate is monitored by the optical fibers to calculate thethermal properties surrounding the TEC/TEF cable.

The disclosed cable can be used in a variety of industrial applications,such as oil and gas downhole surveys, oil and gas downhole permanentinstallations, and non-oil and gas downhole sensing applications such asgeothermal energy or carbon dioxide sequestration monitoring.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the structure of the cabledisclosed herein without departing from the scope or spirit of theinvention. Other embodiments of the invention will be apparent to thoseskilled in the art from consideration of the specification and practiceof the invention disclosed herein. It is intended that the specificationand examples be considered as exemplary only, with a true scope andspirit of the invention being indicated by the following claims.

1. An optical/electrical cable for downhole environments, comprising aplurality of optical fibers disposed within an interior metal tube; anelectrically conducting layer surrounding the interior metal tube; aninsulation layer surrounding and contacting the electrically conductinglayer; and an exterior metal tube surrounding and contacting theinsulation layer, wherein the interior metal tube is coaxial with theexterior metal tube.
 2. (canceled)
 3. The cable according to claim 1,wherein the interior metal tube comprises stainless steel.
 4. The cableaccording to claim 1, wherein the exterior metal tube comprises a steelalloy.
 5. The cable according to claim 1, wherein the electricallyconducting layer includes copper.
 6. The cable according to claim 1,wherein a separating layer is provided between the electricallyconducting layer and the interior metal tube.
 7. The cable according toclaim 1, wherein the optical fibers are embedded in a gel.
 8. The cableaccording to claim 1, wherein the optical fibers perform as opticalsensor or as communication optical fiber, and the interior metal tubehouses together optical fibers performing as optical sensor and opticalfibers performing as communication optical fiber.
 9. A hybrid cable witha coaxial construction, comprising: an exterior metal tube; a layeredtubular conductor positioned coaxially within the exterior metal tube,comprising an electrically conducting layer surrounding and in directcontact with an interior metal tube, where the electrically conductinglayer has an electrical conductivity higher than the interior metaltube; an insulator separating the layered tubular conductor from theexterior metal tube; and optical fibers housed within the interior metaltube, wherein the interior metal tube is coaxial with the exterior metaltube.